1. Field of the Invention
The present invention relates to a manufacturing method of a buried heterostructure semiconductor laser diode and, more in particular, it relates to a burying-regrowing technique of an InGaAlAs type active layer to attain high reliability for long time operation in a buried heterostructure semiconductor laser diode using an InGaAlAs type material for the active layer.
2. Description of the Related Art
Along with increasing use of information communication services, operation communication speed has been made higher and the capacity has been increased in optical communication systems supporting the services. In particular, optical communication equipment having a communication speed of about 10 Gbits per second (10 Gbit/s) has been introduced rapidly not only to public networks such as long distance trunk communication systems or megalopolis communication systems but also to the vicinity of common users such as the Ethernet (registered trade mark). As the high-speed optical communication systems have been extended as far as the vicinity of the users, small-sized and inexpensive optical communication equipments for 10 Gbit/s use have been demanded.
Referring to the optical transmission equipment, an optical module and a light source capable of direct modulation at a high speed of about 10 Gbit/s with no temperature control has been demanded strongly for attaining size reduction and cost reduction. To cope with this demand, semiconductor laser diodes using an InGaAlAs type material for an active layer have been under development as laser diodes that operate at a high-speed of about 10 Gbit/s in a wide temperature range from low temperatures to high temperatures. The structure of the InGaAlAs type laser diodes predominant in the development so far has been of an optical ridge waveguide type because of simple and convenient structure. For example, details are described in xe2x80x9c26th European Conference on Optical Communication (ECOC2000) Proceedingsxe2x80x9d, Vol. 1, p. 123, or the like.
To maximize the utilization of the high-speed characteristics of the laser diode using the InGaAlAs type material as the active layer, it is desirable to introduce a buried heterostructure capable of intensely confining carriers in the horizontal direction. However, in the laser diode using the InGaAlAs type material, since crystal regrowing for burying the active layer with the semiconductor is inhibited by oxidation of Al contained in the active layer, formation of the buried heterostructure is not easy. Al oxides formed in the sidewall of the active layer cause crystal defects in the buried grown layer. The crystal defects cause deterioration of device characteristics and, in the long time device operation, the device characteristics are gradually degraded due to the increase of crystal defects. Therefore, in the buried heterostructure laser diode using the InGaAlAs type material, device reliability for a long time at a practical level has not yet been attained. For example, as described in International conference on Indium Phosphide and Related Materials 1996, Conference Proceedings 1996, pp 765-768, it has been reported in the aging test that operation characteristics are degraded along with long time current supply in a buried heterostructure laser diode using the InGaAlAs type material as the active layer.
To attain the high reliability of such an InGaAlAs type buried heterostructure laser diode, several reports have been presented for the surface cleaning technique in a burying process for a semiconductor laser diode. However, not a few subject have been still left in each of known examples. For example, there can be mentioned JP-A No. 2001-102355 as a first known example. This known example proposes a process of forming a mesa stripe for an Al type active layer by wet etching in a semiconductor laser diode having the active layer and applying a surface treatment with a solution formed by mixing hydrogen fluoride and ammonium fluoride solution. In this process, since the solution treatment is performed, Al on the sidewall of the active layer is oxidized by residual water contained in the solution. Thus, this cannot be said to be a satisfactory method of suppressing oxidation.
Further, in JP-A No. 10-335756 as a second known example, a treatment with chlorine type gas is applied to a mesa stripe formed by dry etching without wet etching treatment. In a case where the mesa stripe is formed by dry etching as in this example, it is necessary to remove damage to the crystal surface caused by dry etching by an appropriate wet etching. Its details are described, for example, in 2001 International Conference on Indium Phosphide and Related Materials, Conference Proceedings 2001, pp 409. Accordingly, also in the case of using the dry etching for the mesa stripe formation step in an InGaAlAs type BH laser diode, wet etching is necessary after dry etching for attaining high reliability.
Further, a technique of forming the mesa stripe of the active layer only with a chlorine type treatment in a crystal growing apparatus has been reported in xe2x80x9c24th European Conference on Optical Communication (ECOC 1998) Proceedingsxe2x80x9d 1998, Vol. 1, pp 75 as a third known example. However, a significant subject remains in applying the reported technique to the InGaAlAs type BH laser diode. At first, since this is etching in a gas phase, the etching depth is restricted. As a result, this involves a subject for the formation of the BH structure at a practical level such that the current blocking layer on both sides of the active layer cannot be increased sufficiently in thickness. Secondly, in a case where the InGaAlAs type material is used for the active layer, it is difficult to perform gas phase etching on the InGaAlAs type material, which brings about a problem, for example, that the crystal surface is roughened by long time etching. To form a desirable mesa stripe as the BH laser diode, it requires not only the gas phase etching in the crystal growing apparatus but also an etching technique such as wet etching on the outside of the crystal growing apparatus.
To attain high reliability in long time operation in a buried heterostructure semiconductor laser diode using the InGaAlAs type material as the active layer, the present invention proposes the following manufacturing method as the burying regrowing technique of the InGaAlAs type active layer. Specifically, the method comprises a series of steps including a step of wet etching an active layer comprising an InGaAlAs type material in a mesa stripe shape, a cleaning treatment for the buried regrown surface with a chlorine type gas in a crystal growing apparatus, a step of burying the active layer in contiguous with the cleaning treatment. By cleaning with the chlorine type gas in the crystallizing apparatus, burying-regrowing is possible in a state of removing an oxide layer formed on the sidewall of the active layer. Unlike the solution treatment as described in the first known example, since the residual water content and the concentration of oxygen in the crystal growing apparatus are sufficiently low, more clean boundary can be obtained. Further, since wet etching is applied to the mesa formation, damage due to plasma as in dry etching can be avoided unlike the second known example. Further, since the wet etching undergoes no restriction for the mesa depth, the structure that cannot be realized by the third known example can be realized by the application of the invention. The gas used for removing the oxide is preferably a gas containing a halogen element such as hydrogen chloride (HCl), methyl chloride (CH3Cl), carbon tetrachloride (CCl4) and carbon tetrabromide (CBr4). Since impurities such as Al oxide on the sidewall of the active layer are removed by the gas containing the halogen element, growth of the buried semiconductor layer is not inhibited. As a result, occurrence of crystal defects to the buried semiconductor layer can be minimized to outstandingly improve the characteristics and the long time reliability of the semiconductor laser diode. The surface treatment by the chlorine type gas does not excite the gas by plasmas, etc., unlike the existent dry etching. Accordingly, the crystal surface can be cleaned with no plasma damage, etc. as worried for the dry etching.
As has been described above, the reliability of the BH laser diode using the InGaAlAs type material for the active layer can be attained only by combination of a series of steps, namely, at least wet etching, cleaning treatment in the crystal growing apparatus, and burying growing contiguous thereto.
The invention can provide a buried heterostructure laser diode that uses the InGaAlAs type material for the active layer, which uses less threshold value current and has high reliability.